How to Choose the Right Compact Tankless Water Heater

Tankless Water Heaters have seen huge increases in unit sales over the last 5 years. Many manufacturers are seeing their yearly sales grow by 100% or more and it is estimated that within the next 5 to 10 years more than 50% of all American homes will have Tankless Water Heaters.

The reason for the huge popularity in Tankless Water Heaters, also called “On Demand” Water Heaters, is because, with them, there is no need for long wait times for water to “warm up” when you turn on the hot water. These compact units mount on a wall either inside or outside of the house and supply hot water on demand for as long as you need it.

There is no longer a need for huge water heater tanks that take up needed space and gallons of wasted water, waiting for it to heat up before you can start using it, there is also no need to keep heating water when you don’t even need it.

Tankless Water Heaters come in a variety of sizes and capacities, so how do you find the size that is right for your needs? This article will explain what you need to consider when choosing the Compact Tankless Water Heater that is right for your needs.

Determine the Flow Rate You Require
Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. This means, to choose the water heater that will fill your needs, you need to determine the flow rate and the temperature rise you will need for its application, such as only in a bathroom. It is important to note that you shouldn’t ever try to save money by undersizing your tankless water heater.

First you need to figure out the total number of devices you want to run and their total flow rate. Then, add up their gallons per minute (gpm) flow rate. This will equal the desired flow rate you will want the water heater to produce.

For instance, say you expect to run a hot water faucet with a flow rate of 0.75 gpm and a shower with a flow rate of 2.6 gpm. The flow rate through the compact water heater would have to be at least 3.26 gpm. To reduce the flow rate, install low-flow water fixtures like Moen’s WaterSense faucets.

Determine the Required Temperature Rise Needed
Next you will need to determine the required temperature rise. This is how much you will need to heat the water, coming into the house, to the desired output temperature. To determine the temperature rise, subtract the incoming water temperature from the desired output temperature.

Unless you know the exact temperature of the incoming water, assume that it is 50°F. Using a low temperature assumption will ensure you don’t undersize your tankless water heater, if you live in a warm climate your water temperature will most likely be much higher.

For most uses, you will want your water to be heated to around 105-115°. Since your assumed incoming water temperature is 50°F, you will need a water heater that produces a temperature rise of 55°F.

Choosing the Right Size
An average shower will have a temperature of between 104-106° and uses 2.6 gallons of water. Assuming your water temperature is 40° coming into your home, and you want to produce enough hot water to run 2 showers at the same time, you will need to raise the incoming water temperature from 40 degrees to 105.

You will need a tankless water heater that can produce at least a 60 degree rise in temperature at 5.2 gallons per minute.

Other Considerations
Tankless Water Heaters that are run on gas are able to produce a larger temperature rise per gpm than electric models do. Most Tankless Water Heaters are rated for a variety of inlet temperatures. Typically, a 70° water temperature rise is possible at a flow rate of 5 gallons per minute through gas-filled demand water heaters and 2 gallons per minute through electric ones.